Notes for Observing
Use these notes together with the information given in the
Australia Telescope Compact Array Users Guide
Current Issues web page
1. User Support
Kate Brooks is the Millimetre Astronomy Research Scientist. She is an astronomer and based in Sydney. Kate will help you with your observing proposals, planning your observations and analysing your data. Kate's research interests are massive star formation and star-forming complexes. Kate.Brooks @ csiro.au
If you are a first-time ATCA observer you should request an observing "friend". An ATNF staff member or student will be assigned to provide you with step-by-step help in preparing your schedule files and with data reduction. You may make a request for an oberving friend via your observing proposal. If someone else will be doing the observations for you and they are using ATCA for the first time they should request a friend directly by contacting Angel Lopez-Sanchez (Angel.Lopez-Sanchez @ atnf.csiro.au ) preferably one month before arriving for the observations.
Assistance throughout your observations is provided by the Duty Astronomer (see the Duty Astronomer Roster). In case of an emergency contact the Duty Astronomer.
2. Observing SeasonThe best time for millimetre observing is the APR observing semester from April 01 to September 30 (also referred to as the "winter season"). Millimetre observations may also be scheduled in the OCT observing semester, between October 01 to March 31. If applying for time in this semester then it is recommended you request your obervations to be scheduled in the months October or March or the nightime of the remaining months (if your source LST range permits).
For 3mm observing there is a 60-65% chance of weather suitable for 3mm observing during the winter season. Good conditions start in May and the weather starts falling apart in October. The height of summer is unsuitable.
For more information on how to apply for time see Observing with ATNF Telescopes.
3. Remote Observing
Currently remote observing is not permitted for 3mm ATCA observations.
4. Doppler Corrections
The Compact Array does not carry out online Doppler corrections. If you are observing a spectral line, use the frequency calculator to determine the appropriate frequency to tune the ATCA receivers.
5. Calibrator cycle time
This calculator determines the calibrator cycle time (the amount of time between successive visits to the secondary calibrator) given observer parameters and parameters describing the phase stability. This assumes that self-calibration is not going to be used.
A calibrator cycle time of around 20min is commonly used for 3mm observing. The integration time on the secondary calibrator is commonly 2 min. Increase this to 3 min if there is a frequency change as a couple of cycles can be waisted while the system adjusts attenuators.
6. Paddle cycle timeYou should make a paddle measurement at least every 20-30 min, more often in bad weather or when you have just moved to a new source (but before taking data). The Tsys value is updated after each paddle measurement. Keep track of this value to gauge how quickly the observing conditions are changing and adjust your paddle cycle time accordingly.
7. Primary Flux Calibration
Follow this observing procedure:
- Determine a suitable secondary calibrator for Uranus. This can be done by first checking the ephemeris of Uranus and then searching for a nearby (< 10 deg) bright (> 1 Jy) calibrator.
- Slew to the secondary calibrator first and make a paddle measurement. You must do a paddle measurement prior to observing your flux calibrator in order to set your flux scale properly.
- Make a pointing measurement on the secondary calibrator.
- Make a 2min integration scan on the secondary calibrator.
- Slew to Uranus and make a 10 to 15 min integration scan.
- Return to the secondary calibrator and make a 2min integration scan.
Observations of the primary flux calibrator (preferably Uranus) can be carried out at any time during the observing run. However, the best approach to acheiving a good flux scale is to ensure that the secondary and the flux density calibrator are observed nearly simultaneously and at the same elevation. Although this may at first glance sound difficult if not impossible, generally it is straightforward. Generally there should be a time when the secondary and the flux density calibrator are at the same elevation. It may be that one is rising while the other is setting, and so they are at significantly different parts of the sky (ie significantly different azimuths). Unless the sky is cloudy, being at different azimuths is not important.
Example schedule file
8. Bandpass Calibration
Observations of the bandpass calibrator (e.g. 1253-055, 1921-293 or 0420-014) can be carried out at any time during the observing run, but the sooner the better in case there are problems later in the run (such as bad weather).
The integration time on the bandpass calibrator is commonly 10 min. The main thing to remember about this scan is that it sould have the same correlator setup (frequency and bandwidth) as your science observations. It is also useful to do a paddle scan beforehand so that you can determine the flux correctly, in case a planet is not available or the planet data are bad.
9. Mosaic observations
If you observing several overlapping fields you will probably want to switch between them rapidly so that each field gets similar integration time and u-v coverage. Even if you are only observing one field, you may want to switch between Source and phase calibrator as rapidly as possible, and switching as part of a mosaic removes a minute or so from the overhead (time it takes to set up a scan).
For general information on ATCA's mosaicing mode see MIRIAD User's Guide.
Refer also to the information available on the Current Issues web page and take note of the warning about mosaicing with too short a cycle time.
10. SCHED related issues
Schedule files for 3mm observing are prepared using the SCHED scheduling program.
Disable the averaging option, i.e., set AVERAGING=1. This is because the changes in the differential atmospheric phase between the antennas might very well cause a decorrelation in visibility amplitudes over long integration times.
Users of SCHED are warned against using GLOBAL command to set frequencies because this circumvents the checks in SCHED that guard against the use of "illegal" frequencies.
For reference pointing:
- For the pointing scan use SCTYPE:POINT & POINT:UPDATE
The "UPDATE" option uses the most recent solution. It has the advantage of giving corrections with respect to the previous solution, which can be used to estimate the real pointing accuracy when the same region of the sky is observed for a long period.
- For the calibrator and source scans use POINTING:OFFSET
This will invoke the latest pointing solution.
- For the scan length give a nominal value of 2 min, although this value is ignored. The sequence takes about 5min to complete.
For a paddle measurement use SCTYPE: PADDLE. The source name is not important as the "ATLOD" routine in MIRIAD now discards these data automatically. For the scan length give a nominal value of 2min, although this value is ignored. The sequence takes about 90 secs to complete.
It is not necessary to specify in your schedule file the coordinates of a planet. Only specify the source name. The source name must match the exact name of the planet and must be in lower case only (e.g. "uranus" not "Uranus"). The coordinates are calculated automatically at the time of reading schedule file in CAOBS.
Note that for the paddle and pointing observations once the scan is complete the next scan in the schedule file will be automatically started.
Example schedule file
11. CAOBS related issues
CAOBS requires to be primed for the reference pointing with commands:
- SET POINT_ANTENNAS 12345
Use this command to select which antennas are to be included in the pointing measurements (5 antennas only for 3mm observing)
- SET POINT_IFFLAG 1234
Use this command to include all four IFs
- SET POINT_PATTERN 2
Use this command to set the number of cycles on each offset of the pointing pattern (normally cycle is 10s).
For observations involving the pointing and paddle routines as well as observation of a planet use "start" rather than "track".
12. Notes on attenuators
You will need to check the level of attenuation in the mm system at the begining of your observations. This is tricky and you should request the help of the Duty Astronomer, even if you are an experienced observer, so that your get the most up-to-date advice. Refer also to the information available on the Attenuation Settings web page.Currently the attenuator levels are set with the caobs command "chklev n", where n is the scan number of the calibrator scan. For 3mm observations run this command with paddle in ("set paddle in").
13. Monitoring Phase Stability
The ATCA seeing monitor is a two-element interferometer with an east-west baseline of 230m, tracking the 30.48GHz beacon on the geostationary satellite OPTUS-B3, at an elevation of 60 degrees. The seeing monitor works by taking the difference between to successive phase measurements, computing the standard deviation of this difference, and converted into a path length in microns. Results from the seeing monitor are updated in real time and displayed on MONICA in the control room. More details about the seeing monitor can be found here.
14. Data Reduction
Observers are advised to update their version of MIRIAD. Absolute flux calibration at all frequencies with ATCA should now be applied using the miriad task MFBOOT. This has superceded both tasks PLBOOT and GPBOOT.
Note that for observations after Aug 2006 it is no longer necessary to provide an improved antenna location solution via the "dantpos" parameter in the MIRIAD task "atfix". For observations prior to Aug 2006 the solutions are available here.
website maintained by: Kate Brooks